In mobile communications, automatic repeat request (“ARQ”) is applied to downlink data from a radio communication base station apparatus (hereinafter “base station”) to radio communication mobile station apparatuses (hereinafter “mobile stations”). With ARQ, mobile stations feed back response signals representing error detection results of downlink data, to a base station. To be more specific, the mobile stations perform CRC (Cyclic Redundancy Check) check for uplink data, and, if CRC=OK (no error), feed back an ACK (ACKnowledgment), and, if CRC=NG (error present), feed back a NACK (Negative ACKnowledgment), as a response signal to the base station. These response signals are transmitted to the base station using an uplink control channel, for example, a PUCCH (Physical Uplink Control Channel) and an uplink L1/L2 CCH (L1/L2 Control Channel).
Further, as shown in FIG. 1, studies are underway to code-multiplex response signals transmitted from a plurality of mobile stations by spreading response signals using CAZAC (Constant Amplitude Zero Auto Correlation) sequences and Walsh sequences (see Non-Patent Document 1). In FIG. 1, [W0, W1, W2, W3] represent a Walsh sequence of a sequence length of 4. As shown in FIG. 1, in a mobile station, an ACK or NACK response signal is subject to first spreading to one SC-FDMA (Single Carrier-Frequency Division Multiple Access) symbol by a CAZAC sequence (with a sequence length of 12) in the frequency domain first. Next, the mobile station associates the response signal after the first spreading with W0 to W3 and performs an IFFT (Inverse Fast Fourier Transform). By this IFFT, the response signal spread by the CAZAC sequence of a sequence length of 12 in the frequency domain is converted to a CAZAC sequence of a sequence length of 12 in the time domain. Then, the signal after the IFFT is secondly spread using a Walsh sequence (with a sequence length of 4). That is, one response signal is assigned to each of four SC-FDMA symbols S0 to S3. Likewise, other mobile stations spread response signals using a CAZAC sequence and a Walsh sequence. Different mobile stations use CAZAC sequences of different amounts of cyclic shift in the time domain or use different Walsh sequences.
Here, the sequence length of a CAZAC sequence in the time domain is 12, so that it is possible to use twelve CAZAC sequences of amounts of cyclic shift “0” to “11” generated from the same CAZAC sequence. Also, the sequence length of a Walsh sequence is 4, so that it is possible to use four different Walsh sequences. Consequently, in an ideal communication environment, it is possible to code-multiplex response signals from maximum forty-eight (12×4) mobile stations.
Meanwhile, in mobile stations, CAZAC sequences of different amounts of cyclic shift between mobile stations are used as ACK/NACK reference signals (hereinafter “RSs (reference signals)”), RSs are subject to second spreading using a spreading code (F0, F1, F2) of sequence length of 3. Consequently, in an ideal communication environment, it is possible to code-multiplex maximum thirty-six (12×3) response signals from the mobile stations.
Here, the cross-correlation between CAZAC sequences between varying amounts of cyclic shift generated from the same CAZAC sequence, is zero. Consequently, in an ideal communication environment, correlation processing in the base station makes it possible to separate a plurality of response signals spread by CAZAC sequences of varying amounts of cyclic shift (the amounts of cyclic shift 0 to 11) and code-multiplexed, without inter-code interference in the time domain.
However, a plurality of response signals transmitted from a plurality of mobile stations do not all arrive at the base station at the same time due to the difference of transmission timings between mobile stations, the influence of multipath delayed waves, frequency offset, and so on. For example, when the transmission timing of a response signal spread by the CAZAC sequence of the amount of cyclic shift “0” is delayed from the correct transmission timing, the correlation peak of the CAZAC sequence of the amount of cyclic shift “0” may appear in the detection window for the CAZAC sequence of the amount of cyclic shift “1.” Further, when there is a delayed wave in a response signal spread by a CAZAC sequence of the amount of cyclic shift “0,” an interference leakage due to that delayed wave may appear in the detection window for the CAZAC sequence of the amount of cyclic shift “1.” In these cases, the CAZAC sequence of the amount of cyclic shift “1” is interfered with the CAZAC sequence of the amount of cyclic shift “0.” Consequently, in these cases, the performance of separation between the response signal spread by the CAZAC sequence of the amount of cyclic shift “0” and the response signal spread by the CAZAC sequence of the amount of cyclic shift “1” degrades. Therefore, if CAZAC sequences of adjacent amounts of cyclic shift are used, the performance for separating response signals may degrade.
Therefore, conventionally, if a plurality of response signals are code-multiplexed by spreading of CAZAC sequences, a cyclic shift interval is provided between CAZAC sequences to reduce inter-code interference between CAZAC sequences. For example, studies are underway to use, when the cyclic shift interval between CAZAC sequences is 2, only six CAZAC sequences of amounts of cyclic shift “0,” “2,” “4,” “6,” “8” and “10” or “1,” “3,” “5,” “7,” “9” and “11” for the first spreading of a response signal among twelve CAZAC sequences of the amounts of cyclic shift “0” to “12.” Therefore, if a Walsh sequence of a sequence length of 4 is used for second spreading of a response signal, it is possible to code-multiplex response signals from maximum twenty-four (6×4) mobile stations.
Further, a base station transmits control information for reporting a resource allocation result of downlink data to mobile stations. This control information is transmitted to mobile stations using mobile station-specific downlink control channels including PDCCHs (Physical Downlink Control Channels), downlink L1/L2 CCHs (L1/L2 Control Channels), DL Grant (Downlink scheduling Grant), and so on. Each PDCCH occupies one or a plurality of CCEs (Control Channel Elements). When one PDCCH occupies a plurality of CCEs, one PDCCH occupies a plurality of consecutive CCEs. According to the number of CCEs required to report control information, the base station allocates either PDCCH in a plurality of PDCCHs to each mobile station, and maps control information to physical resources corresponding to the CCEs occupied by PDCCHs, to transmit the control information.
Further, to eliminate the need for signaling for reporting the PUCCHs to use to transmit response signals from the base station to mobile stations, and to use downlink resources efficiently, studies are underway to associate CCEs with PUCCHs on a one-by-one basis. According to this association, each mobile station is able to identify the PUCCH to use to transmit a response signal from the mobile station, from CCEs corresponding to physical resources to which control information for the mobile station is mapped. Consequently, each mobile station maps a response signal from the mobile station, to the physical resources of the PUCCH based on the CCE corresponding to the physical resources to which control information for the mobile station is mapped.
Here, the number of CCEs occupied by a PDCCH varies depending on the modulation scheme and coding rate (MCS: Modulation and Coding Scheme) of the PDCCH. When a mobile station is located distant from the base station and the received quality at the mobile station is poor, the base station lowers the MCS level of the PDCCH (i.e. lowers the M-ary modulation number or the coding rate) while increasing the number of CCEs. Further, when a mobile station is located near the base station and the received quality at the mobile station is high, the base station raises the MCS level of the PDCCH (i.e. raises the M-ary modulation number or the coding rate) while decreasing the number of CCEs. That is, a PDCCH of a low MCS level occupies a large number of CCEs and a PDCCH of a high MCS level occupies a small number of CCEs. In other words, the number of CCEs for a mobile station to which a PDCCH of a low MCS level is allocated, is great, and the number of CCEs for a mobile station to which a PDCCH of a high MCS level is allocated, is small. If a coding rate of the PDCCH is either ⅔, ⅓ or ⅙ and the PDCCH of coding rate ⅔ occupies one CCE, the PDCCH of a coding rate ⅓ occupies two CCEs, the PDCCH of a coding rate ⅙ occupies four CCEs.
Then, studies are underway to transmit, when a plurality of CCEs are allocated to one mobile station in this way, from a mobile station an ACK/NACK response signal using only a PUCCH associated with the CCE of the smallest number among a plurality of CCEs (see Non-Patent Document 2).
An ACK/NACK response signal transmitted as a PUCCH is signal produces long data transmission delay when a detection error occurs, and therefore an ACK/NACK response signal is generally transmitted so that the error detection rate becomes lower. In mobile communication systems using multi cells, as one approach to reduce the error detection rate, studies are conducted for the method of reducing inter-cell interference (inter-code interference) from neighboring cells.
With allocation method of CCEs and response signal resources disclosed in Non-Patent Document 3, studies are conducted for the method of reducing inter-cell interference (inter-code interference) of response signals transmitted at the same time and in the same frequency by allocating response signal resources differently between neighboring cells. To be more specific, as shown in FIG. 2, as for cyclic shift sequence, one of ACK/NACK transmission parameters, Non-Patent Document 3 discloses the method of reducing inter-cell interference by making a cyclic shift sequence used in a cell different between cells (cell #A, cell #B and cell #C in the figure).
Further, according to Non-Patent Documents 2 and 4, uplink ACK/NACK resources are reported without signaling by allocating the ACK/NACK resources to CCEs to which PDCCHs are allocated on a one-by-one basis, and the method of transmitting an ACK/NACK response signal is disclosed when a PDCCH is transmitted using a plurality of CCEs, for example, using ACK/NACK resources corresponding to the head CCE.
Non-Patent Document 1: R1-072315, Nokia Siemens Networks, Nokia, “Multiplexing capability of CQIs and ACK/NACKs form different UEs,” 3GPP TSG-RAN WG1 Meeting #48bis, St. Julians, Malta, Mar. 26-30, 2007
Non-Patent Document 2: R1-072348, LG Electronics, “Allocation of UL ACK/NACK index”, 3GPP TSG-RAN WG1 Meeting #49, Kobe, Japan, May 7-11, 2007
Non-Patent Document 3: R1-073661, Nokia Siemens Networks, Nokia, “Signaling of Implicit ACK/NACK resources” 3GPP TSG RAN WG1 Meeting #49bis, Orlando, USA, Jun. 25-29, 2007
Non-Patent Document 4: R1-073122, Samsung, “Implicit mapping of CCE to UL ACK/NACK resource,” 3GPP TSG-RAN WG1 Meeting #49bis, Orlando, USA, Jun. 25-39, 2007